Offset printing on paper is a widely used printing process. Instead of conventional cellulose paper supports, optionally provided with one or more additional layers, so called plastic or synthetic papers are also available. An advantage of such plastic or synthetic papers is their outdoor usability due to their improved resistance towards moisture.
Synthetic papers may be classified into two different types: one with a fibrous structure comprising synthetic fibers made from for example polyamides, polyester, or polyolefins; and one in which a film is directly extruded from a thermoplastic polymer.
Extruded films typically have a smooth surface. There are no cavities with capillary activity such as between the fibers of cellulose paper or synthetic fiber webs. The combination of a smooth surface, low absorbing power and a non-polar structure often makes it difficult to print on such polymer films: drying times are long, and the adhesion of the printing ink is poor.
Extruded films are typically made from polyethylene, polypropylene or polyester. By the incorporation of “voids” and/or opacifying pigments in for example the polyester film, an opaque plastic paper can be obtained, such as for example disclosed in WO2008040670, WO2008040701, WO2008116869 and WO2008116797.
To improve the printability, dedicated ink receiving layers have been provided on plastic supports. See for example EP-A 1743976, US20060257593, US20040146699, WO2003033577, U.S. Pat. No. 6,300,393 and JP 11-107194, U.S. Pat. No. 5,397,637 and GB2177413.
An example of a synthetic paper for offset printing is disclosed in EP-A 2103736. It comprises an optionally subbed support and a single layer, the single layer having a layer thickness of at least 3 μm, a pore volume of at least 1.2 ml/m2 and comprising at least one porous pigment, at least one latex and at least one water soluble binder. The water soluble binder is a polyvinyl alcohol-polyvinyl acetate copolymer.
It has been observed that while or after printing on synthetic paper, the blanket roller may be contaminated with “dust”, the dust originating from the ink receiving layer. Such a contamination of the blanket roller with dust may result in printing artefacts. Such a contamination of the blanket roller worsen as more prints are made on synthetic paper without cleaning the blanket roller.
As synthetic paper is often used outdoors, the coating has to be as resistant as possible to moisture. Even under moist conditions, the scratch resistance of the ink receiving layer must be sufficient to avoid damage of the printed image upon contact.